Platinum group metals (PGM) occur in mainly two forms, as discrete minerals and in solid solution in base-metal sulfides. PGMs and PGM minerals are often associated with nickel and copper ores. However, this is not always the case. In South Africa, for example, PGMs are recovered from both Merensky and UG-2 ores.
The predominant base-metal sulfides in Merensky ore are chalcopyrite, pentlandite, pyrrhotite and pyrite. Pentlandite, pyrrhotite and pyrite contain various amounts of platinum, palladium and rhodium. UG-2 ore contains a high chromite content (60-90%) along with 5-25% of gangue silicates, orthopyroxene and 5-15% plagioclase. Trace amounts of base-metal sulfides may also be present, mainly interstitially to the chromite grains. The sulfides are mainly pentlandite, pyrrhotite, chalcopyrite, cobalt-pentlandite and millerite. The PGMs are usually associated with the base metal sulfides and are normally included in or attached to the sulfide grains.
The platinum group metals, which includes platinum, palladium, rhodium, osmium, iridium and, are recovered by traditional flotation methods, i.e. crushing, milling and flotation. Many producers, for example in South Africa, re-grind and float the flotation tail in a so-called MF/MF circuit, i.e. mill/float, mill/float.
Of course, the primary objective of these conventional flotation processes is to increase the recovery of PGMs. Unfortunately, however, the conventional processes have several problems. The first of these is the chromite content in the final flotation concentrate. As chromite has a relatively high density and is brittle in nature, it is inevitably over-ground in a milling circuit. This results in fine chromite being entrained in the final concentrate with serious implications in the downstream smelting process when the levels of Cr.sub.2 O.sub.3 are excessive. Indeed, the maximum permissible chromite content in the final concentrate is preferably 3-4% depending upon the smelter.
Conventional flotation processes also have difficulty in separating PGMs while maintaining an acceptable grade. The flotation rates/kinetics of sulfide minerals are slow. Therefore, in order to achieve an acceptable grade/recovery, conventional flotation circuits have extensive stages of cleaning and re-cleaning.
The order of sulfide mineral bulk flotation response in descending order is chalcopyrite, pyrite, pentlandite and pyrrhotite.
Lastly, the effect of talc can vary from mild to severe depending upon the degree of alteration of the ore. Moderate quantities of talc may be handled by the addition of a depressant such as CMC. However, large quantities of talc create serious difficulty.
It is an object of the present invention to overcome at least some of the disadvantages of the prior art or provide a commercial alternative thereto.